The military wing of Hezbollah, the fundamentalist Islamic Party of God, ambushed an Israeli patrol returning from an overnight operation as it passed the village of Shihin in Israel's self-proclaimed "security zone" at dawn Thursday, according to a military spokesman here.

The platoon commander was killed immediately along with six of his soldiers as shrapnel cut through the patrol, the spokesman said. Two more soldiers were wounded, one seriously. The Israelis had spent the night attempting to ambush Hezbollah guerrillas infiltrating the zone.

The deadliest attack on Israeli forces in southern Lebanon in the past five years, the ambush--less than a mile north of the Israeli border--was a direct challenge to Israeli control over the area, and it put at risk the cease-fire arranged by the United States in the region last month.

Later Thursday, another soldier was killed and two more were wounded by twin bombs detonated, like the first, by remote control. One of the two died later. The explosions occurred as troops from the Golani Brigade searched a ravine near the site of the earlier explosion for the guerrillas who carried out that attack.

The toll was the highest since Israel pulled back to the security zone in 1985 after its 1982 invasion of Lebanon and the three-year occupation of southern portions of the country that followed.

Striking back Thursday afternoon, eight Israeli warplanes bombed four Hezbollah positions, three east of the town of Baalbek and another near the village of Janta in the Bekaa Valley near the Syrian border, according to Lebanese officials in Beirut. Baalbek and Janta are the main supply bases for Hezbollah guerrillas in southern Lebanon.

Hezbollah said in Beirut that two of its guerrillas were killed at an antiaircraft site near Baalbek.

The Hezbollah targets, among them the group's Al Fajr radio station, were described by an Israeli spokesman as well away from Lebanese villages.

More Israeli air strikes are likely today, according to local military observers. They speculated that Prime Minister Yitzhak Rabin would seek to punish Hezbollah for such heavy losses, both as a deterrent to future attacks and to reaffirm his determination to ensure Israeli security while negotiating peace with his country's Arab neighbors.

"A war of attrition is continuing in which Hezbollah is trying to test the security zone, and we are determined that the security zone will remain," said Lt. Gen. Ehud Barak, the Israeli chief of staff. He was referring to the nine-mile-wide strip that Israel established in 1985 along the border in southern Lebanon to protect its northern settlements.

"We are like boxers in a ring," Barak continued. "You can't always avoid being hit, but the point of this activity is to hit the other until he, and not you, falls against the ropes."

When guerrillas from Hezbollah and Islamic Jihad, another fundamentalist group, killed seven Israeli soldiers in a week last month after a series of escalating attacks, Rabin launched a massive air, artillery and naval bombardment of southern Lebanon that left 147 people, most of them civilians, dead and drove hundreds of thousands of Lebanese villagers from their homes.

Under a cease-fire negotiated by Secretary of State Warren Christopher, Israel halted the bombardment on the basis of commitments by Lebanon and Syria that the guerrillas would not rocket communities in northern Israel when Israeli forces struck at the guerrillas in southern Lebanon.

That agreement will now be tested, Israeli officials said Thursday, as their forces prepared to hit Hezbollah--but only outside civilian areas--and waited to see whether the guerrillas would abide by the deal struck in Christopher's mediation.

"This operation is a new lesson for Israel," Sheik Naim Qassem, Hezbollah's deputy leader, said in Beirut. "The enemy thought its aggression (last month) would weaken our resistance. . . . The more the pressure is increased on our nation, the more determined our nation becomes to resist. We will continue fighting until our land is liberated."

Although not a party to the Christopher-brokered agreement, Hezbollah has said that it would not attack civilian targets if Israel did not. Both sides understood that their military forces would remain subject to attack.

With the shaky understanding reached by Christopher now in the balance, Israeli Foreign Minister Shimon Peres, visiting the Norwegian capital of Oslo, called the attack "a big tragedy." He added, "We have done our best to calm the situation in south Lebanon, and this news is most alarming."